Coesite and Stishovite in a Shocked Lunar Meteorite, Asuka-881757, and Impact Events in Lunar Surface

Coesite and stishovite in a shocked lunar meteorite, Asuka-881757, and impact events in lunar surface

E. Ohtania,1, S. Ozawaa, M. Miyaharaa, Y. Itoa, T. Mikouchib, M. Kimurac, T. Araid, K. Satoe, and K. Hiragae

aDepartment of Earth Science, Graduate School of Science, Tohoku University, Sendai 980-8578, Japan; bDepartment of Earth and Planetary Science, University of Tokyo, Tokyo 113-0033, Japan; cFaculty of Science, Ibaraki University, Mito 310-8512, Japan; dPlanetary Exploration Research Center, Chiba Institute of Technology, 2-17-1 Tsudanuma, Narashino, Chiba 275-0016, Japan; and eInstitute for Materials Research, Tohoku University, Sendai 980-8577, Japan

Edited* by Ho-Kwang Mao, Carnegie Institution of Washington, Washington, DC, and approved November 22, 2010 (received for review June 30, 2010)

Microcrystals of coesite and stishovite were discovered as inclu- ered to the Earth after this age. The detailed petrogenesis of this sions in amorphous silica grains in shocked melt pockets of a lunar meteorite is given by Arai et al. (7). meteorite Asuka-881757 by micro-Raman spectrometry, scanning We studied the shocked products of a lunar meteorite Asuka- electron microscopy, electron back-scatter diffraction, and trans- 881757 and discovered both coesite and stishovite crystallites in mission electron microscopy. These high-pressure polymorphs of this meteorite based on the micro-Raman spectroscopic observa-

SiO2 in amorphous silica indicate that the meteorite experienced tions, scanning electron microscopy electron back-scatter diffrac- an equilibrium shock-pressure of at least 8–30 GPa. Secondary tion (SEM-EBSD) measurements, and transmission electron quartz grains are also observed in separate amorphous silica grains microscopy (TEM) observations. We estimated the conditions in the meteorite. The estimated age reported by the 39Ar∕40Ar of collision experienced by this lunar meteorite in the early moon chronology indicates that the source basalt of this meteorite based on its shock textures. was impacted at 3,800 Ma ago, time of lunar cataclysm; i.e., the Results heavy bombardment in the lunar surface. Observation of coesite We used a thin section of Asuka-881757 that contains various and stishovite formed in the lunar breccias suggests that high-

shock features such as maskelynite and glass. Asuka-881757 thin GEOLOGY pressure impact metamorphism and formation of high-pressure section is composed of three fragments, and we made detailed minerals are common phenomena in brecciated lunar surface descriptions of two fragments Asuka-881757-2 and 881757-3. altered by the heavy meteoritic bombardment. The back-scattered electron (BSE) images of the two fragments are given in Fig. 1. moon ∣ impact ∣ collision Host and Shock-Induced Minerals. The fragments Asuka-881757-2 he lunar meteorites are unique samples, which provide and 881757-3 contain large grains of pyroxene, maskelynite (ori- Tinformation on the unexplored surface of the moon. The ginally plagioclase), ilmenite, glass perhaps formed by melting high-pressure polymorphs usually reported in impact craters of during the shock events, and small amorphous silica grains. ’ the Earth s surface have not been reported in lunar samples, The detailed descriptions of this meteorite were given in some and differences in impact conditions in the lunar surface have papers (e.g., ref. 4). been suggested by previous workers; i.e., absence of high-pressure Pyroxene grains are typically 1–2 mm in diameter showing polymorphs of silica might be caused by volatilization during elongated shapes toward c axis, and maskelynites are 1–5mm impact events in the high vacuum at the lunar surface (e.g., refs. 1 in diameter. Pyroxene is highly fractured, whereas maskelynite and 2). However, this is an unrealistic interpretation because shows chemical zonings from cores to rims (An94-77Or0-3). Sym- volatilization during impact would require temperatures exceed- plectic intergrowths of silica-fayalite (Fo2-12)-hedenbergite are ing 1,700 °C. In addition, such volatilization would induce frac- observed at the boundaries between pyroxene and maskelynite. tional sublimation of volatile elements like Na and K, for We observed small grains of amorphous silica, typically about example, and much more importantly, isotopic mass-dependent 50–300 μm in diameter. An electron probe microanalyzer fractionation of oxygen and magnesium. These features were (EPMA) analysis showed that it contains Al2O3 in the range never observed in shocked lunar samples. The lunar meteorites of 0.40–0.68 wt.% and Na2O 0.08–0.14 wt.%. Raman spectra contain information on the shock events that were common in the of the silica grains show no peaks but very broad peak suggesting early lunar surface. Asuka-881757 lunar meteorite was discov- that it is an amorphous silica phase. The SEM-EBSD analysis also ered in Antarctica, and was described by some authors (3, 4). This shows no Kikuchi pattern, which is consistent with the Raman lunar meteorite is composed mainly of coarse aggregates of pyr- spectra. oxene, plagioclase (maskelynite), and ilmenite. It shows a variety of shock features such as the existence of maskelynite and glass Identification of Stishovite, Coesite, and Quartz in Amorphous Silica matrix with compositions of mixtures of pyroxene and plagioclase Grains. We observed several amorphous silica grains and found that is clear evidence for melting of both plagioclase and pyrox- many small granular inclusions of coesite in the amorphous silica ene by shock and quenching of the melt mixture. grains. Fig. 2A shows a transmitted microscopic image of a typical The difference in the age determined by 147Sm-143Nd and amorphous silica grain observed in the present lunar meteorite, a 39Ar∕40Ar chronologies (5) indicates that the source basalt of this fragment Asuka-881757-3. Many coesite inclusions with a grain meteorite crystallized at 3,870 Ma and was impacted at 3,800 Ma,

which is the time of the heavy bombardment on lunar surfaces. Author contributions: E.O. and T.A. designed research; E.O., S.O., and M.M. performed Therefore, the shock features recorded in this meteorite might research; S.O., M.M., Y.I., T.M., M.K., K.S., and K.H. contributed new reagents/analytic provide information on conditions during the heavy meteoritic tools; E.O., S.O., and M.M. analyzed data; and E.O. wrote the paper. bombardment in the Moon. The cosmic-ray exposure age of this The authors declare no conflict of interest. meteorite is one million years (6), which suggests that the meteor- *This Direct Submission article had a prearranged editor. ite was exposed in the space perhaps after launching from the Freely available online through the PNAS open access option. lunar surface one million years ago, and the fragment was deliv- 1To whom correspondence should be addresed. E-mail: [email protected].

www.pnas.org/cgi/doi/10.1073/pnas.1009338108 PNAS Early Edition ∣ 1of4 Downloaded by guest on October 1, 2021 Fig. 1. The BSE images of Asuka-881757 lunar meteorites. Two fragments, Asuka-881757-2 and 881757-3 were studied in this work. The amorphous silica grain, No. 1, in a white box, contains coesite and stishovite inclusions. Another silica grain, No. 2, contains quartz inclusions.

size of 1–10 μm were identified in the amorphous silica grains by the micro-Raman spectroscopy (Fig. 2A, Inset) and SEM-EBSD analysis. We applied the SEM-EBSD analysis and measured the Kiku- chi patterns of the inclusions, which are given in Fig. 3A. The Kikuchi patterns shown in this figure were clearly explained by the crystal structure of coesite. The Kikuchi patterns of the adjacent grains shown in crystals 1, and 2 in Fig. 3A indicate that the granular coesite crystals in the adjacent area (typically in an area of about 30 μm × 30 μm) show a common crystallographic orientation suggesting that they are parts of the same crystal. We also conducted TEM observation of the amorphous silica grain containing coesite crystallites (Figs 2A and 3A), and con- firmed existence of both stishovite and coesite crystals as shown in Fig. 4 A and B. These photomicrographs show that microcrys- Fig. 2. Amorphous silica grains No. 1 in the fragments, Asuka-881757-2 and tals of coesite have a rounded shape with a grain size of 300 nm, 881757-3. (A) Amorphous silica grain, No. 1 contains many coesite inclusions whereas those of stishovite have an angular shape with a size of with 1–10 μm in diameter. A Raman spectrum of an inclusion is shown in the 522 −1 100 nm. Such morphology of stishovite indicates that it did not Inset of this figure, showing the typical Raman peak of coesite at cm . Fd, maskelynite with a feldspar composition; Glass, glass showing mixing of crystallize from a melt. Otherwise it should have occurred as two melts with the pyroxene (brown color) and feldspar (gray) compositions. acicular crystals or needles. (B) a BSE image of amorphous silica grain No. 2. Inclusions of quartz were An amorphous silica grain in a lunar meteorite fragment Asu- observed in this silica grain. SiO2, amorphous silica; A Raman spectrum of ka-881757-2 and its high magnification image are given in an inclusion is shown in the inset of this figure, showing the Raman peaks Fig. 2B. We identified quartz inclusions in the amorphous silica of quartz at 208 cm−1 and 464 cm−1. The major Raman peak observed at grains with the grain size of 1–10 μm in this fragment (Figs. 1 464 cm−1 is larger than 456 cm−1 that is typical for the shock compressed and 2B) by the Raman spectroscopy and SEM-EBSD analysis. quartz (8), suggesting that the quartz grains were formed by the back trans- Fig. 3B shows the Kikuchi patterns of quartz determined by the formation from high-pressure phases. SEM-EBSD measurement of the inclusions. The Kikuchi patterns indicate that the inclusions are quartz and the adjacent iron-sulfide spheres and removed during polishing. In the former crystals in an amorphous silica grain show a common crystallo- case, the vesicles indicate that melting of the meteorite took place graphic orientation as shown in this figure. We observed the at ambient pressure. Raman peak of the quartz inclusions at 464 cm−1, which is at This is a set of high-pressure polymorphs in lunar meteorites, higher wave number than that of the shocked quartz observed at such as stishovite and coesite, in addition to quartz with charac- 456 cm−1 (8), thus strongly suggesting that this quartz originated teristic textures, although we also reported coesite from a CB from partial back transformation of stishovite and/or coesite. chondrite (9). We observed small crystallites of stishovite with a grain size of ∼100 nm in amorphous silica by TEM (Fig. 4B). Discussion Existence of stishovite crystallites in amorphous silica indicates The amorphous silica grains containing coesite, stishovite, and that the peak pressure of the shock compression of the lunar quartz inclusions might have been originally cristoballite formed meteorite was at least 8 GPa (10) and it could have been higher in the final stage of crystallization of the host basaltic magma (1). than 30 GPa due to very low concentrations of Al2O3 (11). No Original plagioclase crystals are converted to maskelynite in occurrence of α-PbO2 type SiO2, seifertite indicates that the this sample. We observed a heterogeneous quench glass formed pressure was lower than ∼35 GPa (12). The amorphous silica by partial melting of both maskelynite and pyroxene and clear might be formed by conversion from former high-pressure poly- evidence for mixing of the two melts as shown in Fig. 2B. The morphs such as stishovite. According to previous studies (13), clear boundary between the amorphous silica grains and the the shock-induced silica glass shows a characteristic defect band quenched pyroxene-feldspar glasses indicates that amorphous at about 602 cm−1, and its intensity decreases by annealing at silica grains were not molten during the high temperature stage high temperatures. Our Raman spectra measurements of the of the formation of the glasses. The spherical holes observed in amorphous silica containing coesite and stishovite show no such Fig. 2B may be formed by vesiculation or originally filled by Raman band corresponding to shocked signature, suggesting

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Fig. 3. Secondary electron images of amorphous silica grain. (A) Amorphous silica grain, No. 1. The Kikuchi patterns of the two coesite grains, 1, and 2 are given in this figure. Two adjacent grains show a similar crystallographic orientation. FIB No. 1 indicates the slice prepared by FIB for TEM observation (see text and Fig. 4). (B) Amorphous silica grain, No. 2. The Kikuchi patterns of the grains 3 and 4 indicate that the two grains are quartz.

formation of the amorphous silica by back transformation from with the preferred orientation of the crystals in the amorphous high-pressure polymorphs. The existence of stishovite and seifer- silica. etite was also reported previously in a lunar meteorite based on The dating of this meteorite was made by Misawa et al. (5), and suggested that the crystallization age is 3871 þ 57 Ma, which the cathodoluminescence studies alone (14), although detailed 147 143 description of their occurrence has not been provided, suggesting was conducted by the Sm- Nd dating, whereas the impact 3798 þ 12 39 ∕40 that impact in the lunar surface may be exceeding the stability age is Ma dated by the Ar Ar chronology. The 39 ∕40 field of stishovite; i.e. >30 GPa. The other high-pressure phases Ar Ar dating that can be modified easily by the shock events such as wadsleyite and ringwoodite were reported previously was made separately both for shocked glass and maskelynite in this meteorite, and the dates were reported to be 3790þ (15) although their occurrences were not described in detail. 16 Ma, 3808 þ 18 Ma respectively (5). The similar ages for the Existence of these minerals is consistent with the shock pressure two parts of this meteorite strongly suggest these parts were estimated here. shocked by a similar collision. The mean age of the glass and The preferred orientation of coesite and quartz crystals ob- the maskelynite, 3798 þ 12 Ma by the 39Ar∕40Ar chronology is A B served by the SEM-EBSD measurement (Fig. 3 and ) suggests considered to be the age of formation of the glass and maskely- that they are formed by topotactic relations with the preexisting nite reset by a large shock event in the lunar surface. Because no crystals such as stishovite. The major Raman peak of quartz at 39Ar∕40Ar ages younger than this age were measured in this −1 464 cm of in the amorphous silica indicates that it was not meteorite, coesite and stishovite in amorphous silica grains in formed by the shock compression but was formed by the back the shock-induced glass are considered to be the products of this transformation during decompression (8), which is consistent collision occurred in the early lunar surface 3798 þ 12 Ma ago,

Fig. 4. TEM images. (A) A coesite nano-crystal observed in the amorphous silica grain recovered from the sample No. 1. The slice was recovered from FIB No. 1 of the sample in Fig. 3A (Upper). A SEAD pattern clearly indicates coesite. Coe, coesite. (B) Stishovite crystals observed in the same slice, FIB No. 1. A SEAD pattern indicates existence of stishovite. Sti, stishovite.

Ohtani et al. PNAS Early Edition ∣ 3of4 Downloaded by guest on October 1, 2021 and we can safely exclude a possibility of formation of these high- crystallographic orientations were investigated by Thermo Noran pressure polymorphs by the shock event that launched this me- Phase ID EBSD system installed into the Hitachi S-4500 field teorite from the lunar surface in a later stage. We can also exclude emission (FE)-SEM at the University of Tokyo. The accelerating the possibility that these minerals formed at the impact on the voltage of the incident beam was 15 kV,and the beam current was ’ Earth s surface. Because a shock vein was cut by fusion crust, 2–3 nA. Calculations of Kikuchi patterns and analyses of the ob- the formation of shock vein and high-pressure minerals evidently served patterns were performed using a program by Kogure (17). predated the atmospheric entry (16). Mikouchi et al. (18) reported the detailed analytical conditions The shock-induced high-pressure minerals in lunar samples for EBSD used in this work. have been so far ignored as important mineral constituents in The distribution and textures of the high-pressure minerals lunar surface regolith breccias experienced by heavy collisions were investigated by optical microscope and an FE-SEM, JEOL during the early lunar history (1, 2). The present occurrence JSM-71010, at Tohoku University in Sendai, Japan. Accelerating of several polymorphs of silica such as coesite, stishovite, and ∼1 quartz in amorphous silica grains suggests that high-pressure voltage and electron-beam current are 15 kV and nA, respec- minerals might be common constituent minerals in lunar surface. tively. The chemical compositions of minerals were determined The high-pressure silica assemblage and the evidence for partial by the electron probe microanalyzer (EPMA; JEOLJXA- back transformation of one or both of them to secondary quartz 8800M superprobe). Analyses were acquired using a 15 kVaccel- allowed to uncover the phase transformations both during the eration voltage and a 5–15 nA beam current. The electron beam dynamic compression and the decompression stages. We empha- was focused to less than 1–10 μm. A dedicated liquid-N2 cooling size that further studies on high-pressure polymorphs in lunar re- sample stage was employed for amorphous silica, maskelynite, golith may be needed by using advanced microanalyses. and glass to reduce electron-beam damage. Slices of the sample for TEM observations were prepared by Experimental Methods the focused ion beam (FIB) system, JEOL JEM-9320FIB and We identified primary, shock-induced, and secondary minerals FEI Quanta 3D. A gallium ion beam was accelerated to 30 kV and documented their spatial distributions in this meteorite by during the sputtering of the slices. The slices were ∼100 nm in using the micro-Raman spectroscopy. We also used an optical thickness. A JEOL JEM-2010 transmission electron microscope microscope, SEM, and TEM to identify the shock texture in this meteorite. Raman spectra of the constituent minerals in operating at 200 kV was employed for conventional TEM and the meteorite were measured by the JASCO NRS-2000 micro- selected area electron diffraction. Raman spectrometer with a nitrogen-cooled CCD detector at Tohoku University. A microscope was used to focus the excitation ACKNOWLEDGMENTS. This work was supported by Japan Society for the laser beam (514.5 nm lines of a Princeton Instruments Arþ laser). Promotion of Science Grants (E.O. and S.O.), and conducted as a part of – Global Center for Excellence program for Earth and Planetary Sciences, The sample was excited by the laser power of 8 12 mW for Tohoku University. A part of this work was supported by the “Nanotechnol- collecting the Raman spectra. The size of the laser beam was ogy Support Project” of the Ministry of Education, Culture, Sports, Science, 1 μm in diameter. The crystalline nature of minerals and their and Technology (MEXT), Japan.

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